The attainment of high-temperature superconductivity with a new class of superconducting materials was of immense scientific and technological importance. Many members of this new class of superconducting materials belong to the family of ceramics called "perovskites". Typically, perovskites are described by the general formula ABX.sub.3 and consist of cubes made up of three distinct elements which are present in a 1:1:3 ratio. The perovskite structure is similar to the naturally occurring calcium titanate structure, CATiO.sub.3, characterized by at least one cation much larger than the other cation or cations. Also included in this family of ceramics are the tungsten bronzes, NaWO.sub.3, strontium titanate, barium titanate, YAl0.sub.3, LaGaO.sub.3, NaNbO.sub.3, KNbO.sub.3, KMgF.sub.3, KMiF.sub.3 and KZnF.sub.3, among others. In the perovskite structure the larger ions (La.sup.+3 =1.15 angstroms, Ba.sup.+2 =1.35 angstroms, and O.sup.+2 =1.40 angstroms) form a cubic close packed structure, with the smaller ions (Cu.sup.+2 =0.96 angstroms, Y.sup.+3 =0.09 angstroms) occupying octahedral interstices in an ordered pattern. Together they form a cubic close packed (face centered cubic) array.
In late 1986, the superconducting properties of certain ceramic defect oxide type materials, which materials are variations of the typical perovskite class of inorganic structures, were observed by Bednorz and Mueller. The Bednorz and Mueller work was based upon materials developed by Michel and Raveau. The materials which Bednorz and Mueller observed contained lanthanum, barium copper, and oxygen, and were reported to be superconducting at a temperature of about 30 degrees Kelvin. Continued work in the field resulted in the increase of the critical temperature, T.sub.c (the temperature at which electrons or holes are able to move through a material without encountering any resistance to that motion), by the substitution of yttrium for lanthanum. Upon analysis, the superconducting composition was found to be a perovskite ceramic defect oxide of the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 type, an orthorombically distorted perovskite. Further work with this phase effectively raised the critical temperature to a few degrees above 90 degrees Kelvin (a temperature above the atmospheric boiling point of liquid nitrogen).
The ceramic defect oxide perovskite phase, having the general composition EQU M.sub.1.sup.IIIA M.sub.2.sup.IIA M.sub.3.sup.IB O.sub.y
was identified utilizing electron microprobe analysis, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. This ceramic defect oxide perovskite, phase was characterized as having a recurring crystallographic unit cell structure including substantially parallel a and b planes spacedly disposed along and substantially parallel to the c-axis thereof.
More specifically, the central plane is a plane of the M.sup.IIIA O type, as a Y--O or La--O plane, with the Group IIIA Metal being surrounded at its four coplanar corners by oxygen sites, which may be occupied or vacant. Immediately above and below this M.sup.IIIA -O plane are equivalent M.sup.IB -O planes of the second type, i.e., Cu--O planes, which the Group IB metal ions being at the four corners of the plane, and occupied oxygen sites being along each edge of the planes. These square planar M.sup.IB atoms (or ions), each surrounded by four oxygen atoms (or ions) have been reported to be critical to superconductivity in the defect oxide perovskites. A pair of M.sup.IIA -O planes are disposed between M.sup.IB -O planes, as shown in FIG. 1, with the first type M.sup.IB -O planes disposed on opposite sides thereof relative to the second type M.sup.IB -O planes. As mentioned above, the vacancies (unoccupied sites) reported to reside in the first type M.sup.IB -O planes are the result of the requirement of electrical neutrality. While the vacancies are generally reported to be in the M.sup.IB -O planes, they may also be in the other planes, as in the M.sup.IIA -O planes. In fact, oxygen is totally absent in the (yttrium) planes.
Other perovskite phase materials have a recurring crystallographic unit cell structure including substantially parallel a and b planes spacedly disposed along and substantially parallel to the c-axis thereof. These other perovskite phase materials include many compositional variations over the basic ABX.sub.3 formula. Such compounds as magnesium-iron silicate, calcium uranium oxide, Ca.sub.2 FeTiO.sub.y, and Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.y are further examples of perovskite phase materials.
The superconducting perovskite type materials are ceramic based defect oxides. That is, the superconducting phases of the perovskite type materials are solids in which different kinds of atoms occupy structurally equivalent sites, and where, in order to preserve electrical neutrally, some sites are unoccupied, or vacant. Since these vacancies may be filled with mobile oxygen atoms, only local order is prevalent with periodicity existing along the planes. These vacant sites form lattice defects, which defects have, generally, profound affects upon the electrical parameters of the superconducting material and more particularly upon the oxidation states of the copper atoms in the unit cells thereof.
Heretofore, single crystal superconducting perovskite type films could only be grown on a "template," i.e., an underlying substrate characterized by substantially the identical crystallographic lattice structure as that of the superconducting film. The superconducting film deposited on this "template" can thereby be epitaxially grown according to the lattice structure of the substrate. Materials, such as strontium titanate and lanthanum aluminate, which have lattice structures matched to the lattice structures of perovskites, are thus utilized as preferred substrates for the epitaxial growth of superconducting perovskite ceramic-oxide type films. However, because these perovskite substrates are very expensive and provide limited surface area upon which to deposit the superconducting material, they remain laboratory curiosities and have limited practical commercial importance.
Sapphire (Al.sub.2 O.sub.3) represents an inexpensive substrate characterized by excellent dielectric properties that would be an ideal substrate upon which to deposit superconducting films. Researchers have recently experimented with the deposition of thin films of YBa.sub.2 Cu.sub.3 O.sub.7 superconducting material atop Al.sub.2 O.sub.3 by laser ablation techniques; however, since Al.sub.2 O.sub.3 does not have a crystallographic lattice structure matching the lattice structure of the depositing YBa.sub.2 Cu.sub.3 O.sub.7 material, these efforts required the prior deposition of a buffer layer having a matching lattice structure to serve as a template for the epitaxial growth of superconducting material.
An article in the American Institute of Physics, Applied Physics Letters Volume 56, No. 8, pp. 785-787, Feb. 19, 1990, by Char et al, entitled "Properties of Epitaxial YBa.sub.2 Cu.sub.3 O.sub.7 Thin Films on Al.sub.2 O.sub.3 " evidences an attempt to deposit superconducting YBa.sub.2 Cu.sub.3 O.sub.7 material directly on Al.sub.2 O.sub.3 substrates. Even a cursory perusal of that publication reveals that the deposited superconducting YBa.sub.2 Cu.sub.3 O.sub.7 film could not be truly epitaxially grown. Notably, the discrete crystallites of the deposited YBa.sub.2 Cu.sub.3 O.sub.7 film identified therein were not aligned as the full width at half maximum of the rocking curve depicted therein was about 2.5.degree.. In fact, relatively wide dispersions in c-axis and in plane alignments of the superconducting material were noted and found to represent mismatched, nonorthogonal axes present in the surface of the sapphire substrate, vis-a-vis, the superconducting material deposited thereupon. It should therefore be evident that these researchers looked to the lattice of the Al.sub.2 O.sub.3 substrate to align the axes of the multi-grained superconducting film, and, as stated therein, Char, et al were attempting to overcome the problem of the large lattice mismatch by growing a buffer layer with better crystallographic lattice structure relative to that crystallographic lattice structure of the superconducting YBa.sub.2 Cu.sub.3 O.sub.7 film. As a matter of fact, in that reported attempt of Char, et al to grow said superconducting material directly atop the substrate, the Al.sub.2 O.sub.3 material was carefully cut along the [1012] plane thereof so as to obtain the best possible lattice match of the sapphire as compared to the superconducting film. Despite this careful attempt at matching lattice structure, Char, et al noted in conclusion that a buffer layer of a more clearly matched crystallographic structure was required in order to grow truly aligned YBa.sub.2 Cu.sub.3 O.sub.7 material atop Al.sub.2 O.sub.3 substrates.
In fact, these same researchers published a second article in July 1990 following up their initial findings. This second article was published in the American Institute of Physics, Applied Physics Letters, Volume 57, No. 4, pp. 409-411, Jul. 23, 1990, by Char, et al, entitled "Microwave Surface Resistance of Epitaxial YBa.sub.2 Cu.sub.3 O.sub.7 Thin Films on Sapphire". The reported research centered on producing epitaxially grown YBa.sub.2 Cu.sub.3 O.sub.7 films on 500 Angstrom thick SrTiO.sub.3 buffer layers which were deposited over Al.sub.2 O.sub.3 substrates. As clearly stated, this research was conducted so as to more precisely align the discrete crystallites of the superconducting film on an insulating, high quality substrate. Moreover, in order to effect such alignment, Char, et al determined that it was necessary to deposit a SrTiO.sub.3 buffer layer which provided the necessary perovskite crystallographic lattice match. Of course, and as one of ordinary skill in the art can readily appreciate, the deposition of a SrTiO.sub.3 buffer layer represents a costly, time consuming procedure which would not have been resorted to had Char, et al been satisfied with the degree of crystallographic alignment present without that buffer layer.
Typically, the non-epitaxially grown superconducting perovskite ceramic defect oxide-type films are polycrystalline in nature, i.e., formed of individual superconducting grains columnarly arising from the underlying substrate (See FIG. 10). In prior work, efforts of the instant inventors at aligning these individual grains have resulted in spatial alignment only along the c-axis of the unit cells thereof (See commonly assigned U.S. patent application Ser. No. 442,380 filed on Nov. 28, 1989, entitled "Method of Aligning Grains of a Multi-Grained Superconducting Material,", pending, the disclosure of which is incorporated hereby reference). While such c-axis alignment provided increased current flow as compared to randomly oriented superconducting material, it failed to provide the high current carrying capacity originally anticipated. While the reasons that c-axis alignment of the unit cells of the superconducting material failed to provide the high current carrying capacity originally anticipated will be detailed in subsequent paragraphs, FIG. 10 illustrates the type of columnar growth present in typical polycrystalline superconducting material characterized by such c-axis orientation. It should be immediately apparent to the informed reader that current flowing along the a-b plane cannot travel very far before encountering the high angle grain boundaries separating adjacent crystallites, which high angle grain boundaries effectively restrict current flow thereacross.
The instant inventors also previously disclosed Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 superconducting films which were modified by the addition of a "parametric modifier" element to fill structural vacancies. (See commonly assigned U.S. Pat. No. 5,004,725 and entitled "Parametrically Modified Superconductor Material," the disclosure of which is incorporated herein by reference.) These researchers at Energy Conversion Devices, Inc. realized that in order to achieve yet higher critical temperatures, it would be necessary to develop a superconducting material in which the chemistry thereof was engineered so as to alter the local chemical and electrical environment. For example, it has been established that the mobility of oxygen atoms in the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 ceramic based systems is very high and therefore the location of those mobile oxygen vacancies at any point in time contribute to the presence or absence of high T.sub.c superconducting phases. It is this oxygen mobility and changing local environment which results in the unstable electronic properties of this class of superconducting materials. Energy Conversion Devices, Inc. found that the addition of the very small and highly electronegative fluorine atoms effectively occupied lattice sites in the ceramic based fluoro-oxide class of superconducting materials so as to cause "grid lock" and provide an impediment to the mobility of oxygen atoms. The result was a stabilized high critical temperature superconducting material. Zero resistance evidence was provided of superconducting phases in "parametrically modified" materials as high as 155 to 168 Kelvin. Magnetic measurements also indicated the presence of yet higher temperature superconducting phases of said fluorinated superconducting material.
The instant disclosure continues to refer to the "parametric modifier" as an agent capable of modifying the local environmental and/or the local chemistry of the superconducting material in such a manner as to affect one or more parameters which control the level of the critical temperature of the superconducting phase. The parametric modifier may also affect the otherwise shielded orbitals of adjacent atoms of the unit cell, in particular the d orbitals. The parametric modifier can act to produce changes in certain parameters which positively affect the critical temperature, while avoiding otherwise related adverse changes in other parameters which would negatively affect the critical temperature. Thus, normally dependent parameters are uncoupled form one another allowing for human engineering of critical features. The parametric modifier can also serve as a catalytic agent to promote grain alignment and to promote film growth along the a-b basal plane. In summary, fluorine, the parametric modifier element of choice, can act in at least one of three ways to improve superconducting properties in the perovskite, defect oxide class of high T.sub.c materials: (1) fluorine can be incorporated in the superconducting material itself; (2) fluorine can act in the plasma to organize the crystallographic growth of the superconducting material; and/or (3) fluorine can promote basal plane growth and inhibit c-axis growth.
The ability of high T.sub.c superconducting materials to carry high critical current densities is not only of great scientific importance, but has immense economic significance. Initially, researchers were not sure of the current density carrying capabilities of the high critical temperature phases of these high T.sub.c superconducting materials. However, this doubt was resolved by scientists at various laboratories throughout the world who demonstrated experimentally that the high T.sub.c ceramic defect oxide superconducting materials could carry current densities exceeding 10.sup.8 amperes per square centimeter at 77 k. This was determined by measuring the current density carried by either a single crystal or an epitaxially grown thin film of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 material in a direction of movement parallel to the a-b plane, i.e., perpendicular to the c-axis of the unit cell thereof. However, the single crystal and the epitaxial thin films were found to be strongly anisotropic and could only carry about 10,000 amperes per square centimeter of current in a direction other than along said basal planes.
These experiments indicate that the high T.sub.c grains of the polycrystalline superconducting material are highly unaligned and the current density is limited by the high angle grain boundaries which result from columnar growth of the relatively small grains thereof. This is contrary to previous thinking to the effect that the alignment of the discrete grains of the polycrystalline superconducting material only along the c-axis, vis-a-vis, the basal plane, would be sufficient to produce materials having high current carrying capacities. It is now clear that alignment of the unit cells in the direction of the a-b axis as well as in the direction of the c-axis of the superconducting material is required in order to obtain an aligned current path and provide a superconducting material capable of carrying high current densities.
The extremely anisotropic nature of the high critical temperature superconducting materials, where the current flows preferentially along the Cu--O plane, and the strong chemical reactivity of the material have been the major stumbling blocks in the commercial development of high T.sub.c superconducting materials. It is clear that, randomly oriented polycrystalline film, tape or wire cannot be utilized to carry the high current densities necessary for most commercial applications. Up to now, the high current carrying capability of the high T.sub.c superconducting materials has only been demonstrated with tiny single crystals or on films epitaxially grown on perovskite substrates characterized by a lattice mismatch of less than 2%, such as SrTiO.sub.3, LaAlO.sub.3, LaGaO.sub.3, etc. However, these substrates are too costly for use in the fabrication of commercial devices, are available only in small wafers, and/or possess high dielectric constants and high dielectric losses. Further, such free standing single crystal superconducting materials are many times too small, inherently brittle and inflexible. Accordingly, flexible, epitaxial-like films grown on electronic quality, inexpensive substrates must be utilized in order to make it commercially feasible to fabricate wire or other flexible superconducting material. Further, the routineer will appreciate that in numerous commercial applications, it is necessary that the superconducting material be grown on top of a metallic, highly conductive substrate, such as copper, silver or gold, to avoid a catastrophic failure in the event the superconducting material reverts back to its normal state.
Therefore, an urgent and long felt need has existed for a method of growing single crystal, epitaxial or epitaxial-like thin films of substantially flexible high T.sub.c superconducting material on inexpensive, non-perovskite, good dielectric substrates, such as sapphire or a noble metal, especially if those superconducting films are to be capable of 1) providing high current carrying capacities and 2) being grown in an inexpensive, roll-to-roll, mass production process. It is to satisfy these crucial and long felt needs in the art that the instant invention is directed.